A transmission unit of this kind can be used in various applications in order to provide two different transmission ratios, between which it is possible to switch by controlling the throttling device. By completely shutting off the throttling device, for example, it is possible to block the pump and hence to set the planetary wheel set to rotate as a unit. By opening the throttling device, circulation of the hydraulic fluid between the suction zone and the pressure zone can be enabled, thereby decoupling the corresponding gearwheel elements of the planetary wheel set from one another, apart from the flow resistance.
The throttling device can be a valve arranged in the vicinity of the pump casing. In order to connect the suction zone and the pressure zone of the pump to the throttling device, suitable suction and discharge lines can be provided. In such arrangements, however, the required installation space for the transmission unit is increased. Moreover, depending on length and line cross section, the suction and discharge lines cause a relatively high flow resistance with consequent losses of power.
U.S. Pat. No. 3,724,975 and Great Britain Patent No. 465,363 disclose rotary piston pumps which contain gearwheel elements for pumping hydraulic fluid from a suction zone to a pressure zone of a pump casing containing the planetary wheel set (22), and provide a controllable throttling device in order to selectively throttle a fluid flow pumped between the suction zone and the pressure zone.